(a) Field of the Invention
The present invention relates to a liquid crystal display and a method of driving the same and, more particularly, to a liquid crystal display which has a function of making adaptive color correction.
(b) Description of the Related Art
As personal computers and televisions become thinner and flatter, flat panel type display devices such as liquid crystal displays have developed, and have been employed for practical use in various fields instead of cathode ray tubes.
The liquid crystal display has two substrates, and a liquid crystal sandwiched between the two substrates with a property of dielectric anisotropy. In operation, an electric field is applied to the liquid crystal while being controlled in strength thereof. In this way, the light transmission through the liquid crystal is controlled to thereby display the desired picture images.
Such a liquid crystal display exhibits the so-called inter-gray scale color shift phenomenon in various modes such as TN and ECB.
First, in the modes of TN, ECB and CE, the light transmission is determined by the following mathematical formulas 1 to 3, respectively.T=1−((sin2(π/2√(1+u2))/1+u2), for TN  (1)
where u=2Δnd/λ.T=½ sin2(πΔnd/λ)=½ sin2((π/2)u), for ECB  (2)T=sin2(2θ)sin2((π/2)u), for CE  (3)
In the mathematical formulas 1 to 3, with the variation in voltage, the value of u being in inverse proportion to the wavelength is altered in the case of TN or ECB mode, while the value of θ is altered in the case of CE mode.
That is, in case the liquid crystal molecules are aligned in the vertical direction while being altered in the effective value of Δnd, the light transmission is differentiated per each wavelength bearing intrinsic diffusion characteristics. This is expressed in the mathematical formulas 1 and 2 with the presence of λ at the denominator of u.
By contrast, in the case of CE mode, the light transmission is not differentiated at the respective wavelengths even if the driving voltage is varied.
FIG. 1 is a graph illustrating the difference in light transmission at the wavelengths of 450 nm and 600 nm as a function of Δnd in the TN and ECB modes. The maximum values of light transmission at the ECB and TN modes are about 0.27 nm and 0.47 nm, respectively. Such light transmission values are divided by the value of X.
As shown in FIG. 1, since the light transmission at lower wavelengths becomes higher with the middle gray scales in the TN and ECB modes, the graph is protruded in the direction of plus (+), and this inclination is somewhat stronger in the ECB mode than in the TN mode. For this reason, the inter-gray scale color shift phenomenon becomes serious in the ECB or TN mode.
FIG. 2 is a graph illustrating the graph values of FIG. 1 divided by the light transmission.
As shown in FIG. 2, blue sensation is made at the low gray scales, while the color sensation becomes yellowish at the higher gray scales.
The inter-gray scale color shift phenomenon is generated to be more serious in the VA mode than in the TN mode. The color shift phenomenon is relatively weak in the TN mode compared to the VA mode due to the effect of light revolution where the light transmitted through a target material is rotated by a predetermined angle with respect to the polarizing surface for the incident light.
In the presence of such a color shift phenomenon, color sensation is altered depending upon the gray levels.
FIG. 3A illustrates the color sensations per gray patterns, and FIG. 3B illustrates the color sensations per gray patterns in a usual PVA mode liquid crystal display.
As shown in the drawings, the bright grays involve much of the red content, and the dark grays involve much of blue content. Accordingly, even in the display of an arbitrary middle gray scale, it appears to be more bluish while coming towards the dark gray. In case a personal face is displayed, the blue-based color sensation is made while producing a feeling of coldness.
The reason that such a difference in color sensation is made can be found through measuring gamma curves of R, G and B in a separate manner.
FIG. 4 is a graph illustrating the variation in color coordinates per white grays in the PVA mode. As known from the graph, the movement range of the color coordinates of white grays is very great.
FIG. 5 is a graph illustrating the color temperatures per usual grays. The color temperature refers to the temperature of a black body irradiating the light of the same color coordinates as the light from a light source.
In gray scale expressions, it is ideal to have a constant color temperature irrespective of increase or decrease in the gray levels. However, as known from the graph of FIG. 5, the actual situation is that the color temperature is radically elevated while coming towards a dark level (or a black level).
FIG. 6 illustrates the RGB gamma curves in a usual PVA LCD panel. Of course, the brightness levels per grays in the RGB gamma curves are differentiated, but normalized in the drawing.
As shown in FIG. 6, the RGB gamma curves are not agreed to each other while being differentiated in distance. That is, as it comes toward the dark gray level, the G content or the R content is approximated to zero, and only the B content involves a brightness level higher than zero. Consequently, the screen image appears to be very bluish to the eye of the beholder.